One feature common to all computed tomography systems is that they must include apparatus for detecting radiation intensity variations indicative of a patient's internal structure. In a first generation CT system this apparatus involved a single detector which moved in unison with a moving X-ray source. Second and third generation CT systems included a plurality of detectors also moving with respect to the patient and in a fourth generation system a high number of detectors are fixed with respect to a patient.
Early computed tomography systems used one of two types of X-radiation detectors. One type detector is the scintillation detector which uses a crystal for converting X-radiation impinging upon it into visible light which is detected by a photomultiplier tube. This tube converted the light into an electrical current and amplified this current to a level suitable for measuring the radiation intensity.
Gas ionization chambers are also used in some CT designs. In such a chamber the high energy radiation passing from the source through the patient to the detector causes an ionization of the gas stored in the ionization chamber. The current generated in this ionization process, gives an indication of X-radiation intensity.
Photodiode solid state detectors have increasingly replaced the photomultiplier tube techniques for detecting light from a scintillation crystal. Detectors now typically comprise a scintillation crystal mounted in close proximity to a photodiode which produces an output current proportional to the X-ray intensity striking the scintillation crystal.
In a fourth generation CT design an array of 600 or more closely packed photodiodes are positioned in a circular array to circumscribe a patient aperture. X-radiation from a rotating X-ray tube impinges upon each detector in the array as the X-ray tube orbits around the patient. Electronics coupled to each detector convert the current output from the detector photodiode into a digital signal proportional to the X-ray intensity impinging upon a given detector.
The solid state detectors have advantages over gas ionization or photomultiplier tube detectors. Perhaps the most significant advantage the solid state detectors have is the very close packing density these detectors permit in a fourth generation machine. A typical scintillation crystal and photodiode is mounted in a package sufficiently small to allow well over 1000 detectors to be positioned in a reasonable space for scanning.
In a CT apparatus requiring in excess of 1000 detectors mounted in a circular array about a subject aperture, it is possible that certain detectors may malfunction and no longer produce meaningful intensity data. It is also possible that the electronic circuitry for analyzing the output from the detectors will also, on occasion, malfunction to the point where the output from the circuitry may be inaccurate or unreliable. In either event, it is desirable that the view resulting from the bad detector or circuit (detector channel) is corrected prior to reconstruction.
In the past when a detector or circuit has malfunctioned, an intensity reading has been assigned to the malfunctioning detector by averaging the intensity outputs from the two detectors closely adjacent to the malfunctioning detector. Experience with this simple averaging technique indicates that it is not a full solution since using a simple averaging on occasions leads to artifacts in the reconstructed picture. These artifacts typically comprise streaks across the reconstructed image which correspond to no internal structure in the subject. These streakings or artifacts occur because incorrect values are assigned to the sample in the averaging technique.
A more sophisticated technique for supplying intensity readings to malfunctioning detectors and/or circuits is disclosed in pending U.S. patent application Ser. No. 441,857 entitled "Method and Apparatus for Computed Tomography Imaging" to DeMeester et al which was filed in the U.S. Patent Office on Nov, 15, 1982, now abandoned.
The DeMeester et al invention selectively chooses an interpolation based upon differences in intensity readings from closely adjacent functioning detectors. This technique can still, however, produce artifacts under certain conditions. In particular high attenuation intensity samples corresponding to a high contrast region within a cross-section can result in an inappropriate choice of interpolation data.